Centrifugal fan

ABSTRACT

A centrifugal fan includes a motor having a rotor, an impeller that is fixed to the rotor, a circuit board electrically connected to the motor, and a casing that accommodates the motor, the impeller, and the circuit board. The impeller includes a cylindrical boss portion fixed to the rotor, a plurality of blade portions that are arranged in a circumferential direction and extend radially outward, an upper shroud having an annular shape and connecting axially upper portions of the blade portions, and a lower shroud having an annular shape and connecting axially lower portions of the blade portions. The casing has a lower casing disposed axially below the impeller. The lower casing has a board accommodating portion for accommodating the circuit board, and a cover member facing a lower end face of the blade portions and an upper face of the circuit board in the axial direction.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to Japanese Application No. 2017-208441 filed on Oct. 27, 2017 the entire content of which is incorporated herein by reference.

FIELD

The present disclosure relates to a centrifugal fan.

BACKGROUND

A centrifugal fan has a structure in which an impeller having a plurality of blades arranged on the circumference thereof is accommodated between an upper casing in which an air intake port is formed and a lower casing. As the impeller rotates, the centrifugal fan discharges air, introduced through an opening portion, to a side of the impeller. The lower casing is made of a metal plate and has a recessed portion recessed downward. A motor is attached to a bottom surface of the recessed portion. A portion of a stator of the motor and a circuit board on which a drive circuit of the motor is mounted are accommodated in the recessed portion. The recessed portion is provided with a hole portion through which a supplier that supplies electric power for rotating the motor passes.

With a configuration in which the blade portion of an impeller is sandwiched between an upper shroud and a lower shroud, in the case where the impeller is formed as a single member, a lateral slide mechanism is required for the mold and the mold structure becomes complicated. There is a restriction that an undercut portion cannot be provided in the vicinity of the air intake port. On the other hand, in the case where the impeller is formed of two members, the geometrical problem that arises when the impeller is formed as a single member is reduced. However, the difficulty of manufacturing becomes high because two molds and a method such as welding for fastening the two members is used.

On the other hand, in high-output motors, as larger electronic components are used, the size of the circuit board has increased in accordance with an increase in the size of the electronic components. In the case where a circuit board is made larger with the configuration of the existing centrifugal fan, the recessed portion for accommodating the circuit board is enlarged. As a result, a space is generated between the peripheral edge of the recessed portion and the impeller, and the wind guiding function of the lower casing is lost. Thus, there is a possibility that static pressure characteristics, air volume characteristics, and noise characteristics deteriorate when the circuit board is increased in size.

As a countermeasure against this deterioration, the space between the peripheral portion of the recessed portion and the impeller is filled by extending the lower shroud of the impeller radially outward. However, in this method, an undercut portion is generated in the upper shroud and the lower shroud, which causes a problem that the mold structure becomes complicated. In addition, the height of the blade portion is shortened and there is a concern that the airflow rate in a thin centrifugal fan decreases.

SUMMARY

A centrifugal fan according to at least one embodiment of the present disclosure includes a motor having a rotor that rotates about a center axis extending vertically. The centrifugal fan further includes an impeller that is fixed to the rotor and rotates together with the rotor. The centrifugal fan further includes a circuit board electrically connected to the motor. The centrifugal fan further includes a casing that accommodates the motor, the impeller, and the circuit board. The impeller includes a cylindrical boss portion fixed to the rotor, a plurality of blade portions that are arranged at intervals in a circumferential direction on a radially outer side of the boss portion and extend radially outward, an upper shroud having an annular shape and connecting at least portions on an axially upper side of the blade portions, and a lower shroud having an annular shape and connecting at least portions on an axially lower side of the blade portions. The casing has a lower casing disposed axially below the impeller. The lower casing includes a board accommodating portion that is recessed axially downward and accommodates the circuit board, and a cover member that faces at least a portion of a lower end surface of the blade portion and at least a portion of an upper surface of the circuit board in an axial direction.

The above and other elements, features, steps, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of some embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external perspective view of a centrifugal fan according to at least one embodiment of the present disclosure.

FIG. 2 is a vertical cross-sectional view of the centrifugal fan according to at least one embodiment of the present disclosure.

FIG. 3 is a perspective view of the centrifugal fan in a state in which an upper casing, an impeller, and a cover member are removed from the centrifugal fan according to at least one embodiment of the present disclosure.

FIG. 4 is a schematic plan view of the cover member according to at least one embodiment of the present disclosure.

FIG. 5 is a perspective view of the impeller according to at least one embodiment of the present disclosure.

FIG. 6 is a perspective view of the impeller according to at least one embodiment of the present disclosure.

FIG. 7 is a partial cross-sectional view around an exhaust port of the centrifugal fan according to at least one embodiment of the present disclosure.

FIG. 8 is an enlarged cross-sectional view of the vicinity of the upper shroud and the lower shroud of the impeller in FIG. 7 according to at least one embodiment of the present disclosure.

FIG. 9 is an enlarged cross-sectional view of a centrifugal fan according to at least one embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, at least one embodiment of the present disclosure will be described in detail with reference to the drawings. Further, in this specification, a direction parallel to the center axis P of a motor 30 provided to a centrifugal fan 100 in FIG. 2 is referred to as an “axial direction”, a direction orthogonal to the center axis P is referred to as a “radial direction”, and a direction along an arc with the center axis P as the center is referred to as a “circumferential direction”. In addition, in the present specification, the shape and positional relationship of each element will be described with the axial direction as an up-down direction, the side of an impeller 10 as an upward direction, and the side of the motor 30 as a downward direction. However, there is no intention to limit the orientation during use of the centrifugal fan 100 according to the present invention to this definition of the up-down direction.

In the present disclosure, a “parallel direction” includes a substantially parallel direction. In addition, in the present disclosure, a “perpendicular direction” includes a substantially perpendicular direction.

FIG. 1 is an external perspective view of a centrifugal fan 100 according to at least one embodiment of the present disclosure. FIG. 2 is a vertical cross-sectional view of the centrifugal fan 100 according to at least one embodiment of the present disclosure. FIG. 3 is a perspective view of the centrifugal fan in a state in which an upper casing 2, an impeller 10, and a cover member 6 are removed from the centrifugal fan 100 according to at least one embodiment of the present disclosure. In FIGS. 1 and 2, the centrifugal fan 100 includes a casing 1, the impeller 10, a motor 30, and a circuit board 40.

The casing 1 accommodates the impeller 10, the motor 30, and the circuit board 40. The casing 1 has an upper casing 2 and a lower casing 3. The upper casing 2 and the lower casing 3 include a resin, for example. However, the upper casing 2 and the lower casing 3 may include materials other than a resin, such as a metal. The upper casing 2 and the lower casing 3 may include the same material or different materials. Further, the centrifugal fan 100 may be configured not to have the upper casing 2.

The upper casing 2 is disposed axially above the impeller 10. The upper casing 2 has an air intake port 2 a facing a radially central portion of the impeller 10. In detail, the upper casing 2 has a rectangular tubular shape in which four corner portions are combined with the outer periphery of a cylindrical portion. The air intake port 2 a is circular. However, the upper casing 2 may have another shape such as a cylindrical shape. The shape of the air intake port 2 a may be other than circular. By providing the upper casing 2, the occurrence of a turbulent flow around the upper shroud 15 described below can be suppressed. Moreover, the centrifugal fan 100 can efficiently send air in the centrifugal direction.

The lower casing 3 is disposed axially below the impeller 10. The lower casing 3 has a rectangular shape in a plan view from the axial direction, and has substantially the same size as that of the upper casing 2. In at least one embodiment, the lower casing 3 has a shape other than rectangular. The lower casing 3 has a board accommodating portion 4 and a cover member 6. The lower casing 3 further has a flange portion 5.

The board accommodating portion 4 is recessed downward axially, and accommodates the circuit board 40. In at least one embodiment, the board accommodating portion 4 has the same shape as that of the circuit board 40, and is slightly larger in the radial direction than the circuit board 40. However, the shape of the board accommodating portion 4 may not be the same as the shape of the circuit board 40. The motor 30 is positioned at a radially center portion of the board accommodating portion 4, and a portion of the circuit board 40 protrudes radially outward with respect to the motor 30.

A flange portion 5 extends radially outward from the outer peripheral edge of the board accommodating portion 4. A portion of the cover member 6 is fixed to the flange portion 5. FIG. 4 is a schematic plan view of the cover member 6 according to at least one embodiment of the present disclosure. In FIGS. 2 and 4, the cover member 6 has a rectangular plate shape and has a circular opening 6 a at the center. The shape of the opening 6 a is not limited to a circular shape, and may be another shape such as a rectangle. In at least one embodiment, the cover member 6 has a stepped portion 6 b in which the thickness in the axial direction varies, and the thickness of the radially inner side from the stepped portion 6 b is thinner than the thickness of the radially outer side from the stepped portion 6 b. The cover member 6 is disposed on the flange portion 5. A portion of the cover member 6 radially outside the stepped portion 6 b is in contact with the flange portion 5, and is fixed to the flange portion 5. A region for fixing the cover member 6 can be secured by the flange portion 5, whereby the cover member 6 can be fixed firmly.

A method of fixing the cover member 6 to the flange portion 5 is not particularly limited. The cover member 6 may be fixed to the flange portion 5 using, for example, screws, or may be fixed with an adhesive. In at least one embodiment, the cover member 6 is fixed to the flange portion 5 by screws (not illustrated) together with the upper casing 2. However, the cover member 6 may be fixed to the flange portion 5 by screws, separately from the upper casing 2. In addition, the cover member 6 may have a flat plate shape not having the stepped portion 6 b. Further, the cover member 6 includes a resin or a metal, for example. The cover member 6 may include the same material as the material constituting the board accommodating portion 4 and the flange portion 5. Furthermore, in at least one embodiment, the radially outer edge of the cover member 6 is at the same position as that of the radially outer edge of the flange portion 5. However, the radially outer edge of the cover member 6 may be positioned radially inside the radially outer edge of the flange portion 5, or may be positioned radially outside the radially outer edge of the flange portion 5.

The motor 30 has a rotor 32. The motor 30 also has a stator 31, a shaft 33, a bearing portion 34, and a bearing holding portion 35.

The rotor 32 rotates about the center axis P extending vertically. The rotor 32 is disposed above the stator 31 on the radially outer side. The rotor 32 has a cup shape opening downward. In at least one embodiment, the rotor 32 has a shape other than a cup shape. The impeller 10 is disposed radially outside the rotor 32, and the impeller 10 is fixed to the rotor 32. The shaft 33 is disposed radially inside the rotor 32, and the shaft 33 is fixed to the rotor 32. A rotor magnet 36 is fixed to the inner peripheral surface of the rotor 32. In at least one embodiment, the rotor magnet 36 is a single annular magnet. N poles and S poles are alternately magnetized in the circumferential direction on the radially inner surface of the rotor magnet 36. Instead of a single annular magnet, a plurality of magnets may be arranged on the inner peripheral surface of the rotor 32.

The shaft 33 is a columnar member disposed along the center axis P. As the material of the shaft 33, for example, a metal such as stainless steel may be used. An upper end portion of the shaft 33 is located above the bearing portion 34 on the upper side. The upper end portion of the shaft 33 passes through a rotor hole axially penetrating along the center axis P of the rotor 32, and is fixed to the rotor 32.

The bearing portion 34 rotatably supports the shaft 33 around the center axis P. In at least one embodiment, the number of the bearing portions 34 is two, and the two bearing portions 34 are lined up and down. The two bearing portions 34 are composed of ball bearings. The number and type of the bearing portions 34 may be appropriately changed.

The bearing holding portion 35 supports the stator 31 radially outside, and supports the bearing portion 34 radially inside. As the material of the bearing holding portion 35, for example, a metal such as stainless steel, brass or the like may be used. However, the material of the bearing holding portion 35 is not limited to a metal and may be a resin. The bearing holding portion 35 extends axially in a cylindrical shape around the center axis P. The lower end portion of the bearing holding portion 35 is inserted into a circular hole around the center axis P provided to the lower casing 3, and is fixed to the lower casing 3.

The stator 31 is an armature that generates a magnetic flux according to the drive current. The stator 31 has a stator core, an insulator, and a coil.

The stator core is a magnetic body. The stator core is formed by laminating electromagnetic steel plates, for example. The stator core has an annular core back and a plurality of teeth. The core back is fixed to the outer peripheral surface of the bearing holding portion 35. The plurality of teeth protrude radially outside from the core back. The insulator is an insulating body. As a material of the insulator, for example, a resin may be used. The insulator covers at least a portion of the stator core. The coil is formed by winding a conductive wire around the teeth via the insulator.

By supplying a drive current to the stator 31, a rotational torque is generated between the rotor magnet 36 and the stator 31. As a result, the rotor 32 rotates with respect to the stator 31, and the impeller 10 fixed to the rotor 32 also rotates about the center axis P. The motor 30 in FIG. 2 is an outer rotor type motor in which the rotor 32 is disposed radially outside the stator 31. However, the motor 30 may be an inner rotor type motor in which the rotor 32 is disposed radially inside the stator 31.

The circuit board 40 is electrically connected to the motor 30. The circuit board 40 is supported on the lower side of the motor 30. The circuit board 40 is disposed in the board accommodating portion 4 of the lower casing 3. The circuit board 40 is disposed substantially perpendicular to the center axis P on the upper side of the lower casing 3 and on the lower side of the stator 31. The circuit board 40 is, for example, fixed to the insulator. An electric circuit that supplies a drive current to the coil is mounted on the circuit board 40. End portions of the conductor forming the coil are electrically connected to terminals provided on the circuit board 40.

FIG. 5 is a perspective view of the impeller 10 according to at least one embodiment of the present disclosure. FIG. 6 is a perspective view of the impeller 10 according to at least one embodiment of the present disclosure. The impeller 10 is fixed to and rotates with the rotor 32. In FIGS. 5 and 6, the impeller 10 includes a boss portion 11, a plurality of blade portions 13, an upper shroud 15, and a lower shroud 17. In at least one embodiment, the boss portion 11, the blade portions 13, the upper shroud 15, and the lower shroud 17 are a single member made of the same resin material.

The boss portion 11 is tubular, and is fixed to the rotor 32. In at least one embodiment, the boss portion 11 is cylindrical, and is fixed to the outer peripheral surface of the rotor 32 above the motor 30. The boss portion 11 is fixed to the rotor 32 by, for example, press fitting or adhesion. In detail, the boss portion 11 has an annular protruding portion protruding radially inward at the axially upper end. The protruding portion is located above the rotor 32. The protruding portion may not be provided. The protruding portion is provided to enable, for example, a weight member for performing balance adjustment to be arranged.

The blade portions 13 are disposed at intervals in the circumferential direction on the radially outer side of the boss portion 11, and extend radially outward. In at least one embodiment, the blade portions 13 face the boss portion 11 in the radial direction via a gap. However, the blade portions 13 may be in contact with the boss portion 11. In a plan view, the blade portions 13 are inclined in the opposite direction of the rotation direction of the centrifugal fan 100, and extend radially outward. The direction in which the blade portions 13 extend is not limited to radially outward. A portion of the blade portions 13 may extend in the same direction as the rotation direction or may extend perpendicularly to the rotation direction. Further, in at least one embodiment, the blade portions 13 are arranged at equal intervals in the circumferential direction. However, in at least one embodiment, the blade portions 13 are at varying intervals.

The upper shroud 15 is annular. In at least one embodiment, the upper shroud 15 is in a ring shape. The upper shroud 15 connects at least axially upper portions of the blade portions 13. In at least one embodiment, the upper shroud 15 connects radially outer portions of the blade portions 13.

The lower shroud 17 is annular. In at least one embodiment, the lower shroud 17 has a cylindrical shape tapered from the axially lower side to the upper side. The lower shroud 17 connects at least axially lower portions of the blade portions 13. In the present embodiment, the lower shroud 17 connects radially inner portions of the blade portions 13. The radially inner edge of the lower shroud 17 is connected to the boss portion 11. The upper shroud 15 and the lower shroud 17 are connected by the blade portions 13.

The air sucked from the air intake port 2 a of the upper casing 2 is spun in the casing 1 in the circumferential direction by the rotation of the impeller 10, and is discharged from an exhaust port 2 b provided between the upper casing 2 and the lower casing 3. The upper shroud 15 and the lower shroud 17 efficiently guide the air taken into the casing 1 from the air intake port 2 a to the exhaust port 2 b, thereby improving the fan efficiency of the centrifugal fan 100. In at least one embodiment, the exhaust port 2 b is provided on the entire circumference of the casing 1. However, the exhaust port 2 b may be provided only in a part of the circumferential portion of the casing 1.

Next, a configuration around the exhaust port 2 b, which is a characteristic part of the centrifugal fan 100 according to at least one embodiment, will be described in detail. FIG. 7 is a partial cross-sectional view around the exhaust port 2 b of the centrifugal fan 100 according to at least one embodiment of the present disclosure. FIG. 8 is an enlarged view of the vicinity of the upper shroud 15 and the lower shroud 17 of the impeller 10 in FIG. 7 according to at least one embodiment of the present disclosure.

In FIG. 7, the rotor 32, and the boss portion 11 and the blade portions 13 of the impeller 10 are disposed so as to overlap in the radial direction. The lower end surface of the rotor 32 and the lower end surface of the boss portion 11 are positioned at an axially upper side from a lower end surface 13 a of the blade portion 13. Further, the axial height of the boss portion 11 is accommodated within the axial height of the impeller 10. By arranging components in this manner, the centrifugal fan 100 is thinner than other arrangements. Further, in at least one embodiment, the blade portion 13 extends to a position where a part of the blade portion 13 overlaps the flange portion 5 in a plan view from the axial direction. Therefore, the amount of air generated by the rotation of the impeller 10 can be increased.

In FIG. 8, the radially outer edge 17 a of the lower shroud 17 is located at the same position in the radial direction as that of the radially inner edge 15 a of the upper shroud 15 a. As a result, when the impeller 10 is formed by injection molding with a resin, the mold is removable in one direction such at the up-and-down direction. Therefore, as there is no need to use a split mold or to provide a slide mechanism to the mold, the structure of the mold or the manufacturing process can be simplified. The radially outer edge 17 a of the lower shroud 17 may be positioned radially inside the radially inner edge 15 a of the upper shroud 15. That is, as long as the radially outer edge 17 a of the lower shroud 17 is located at the same position in the radial direction as that of the radially inner edge 15 a of the upper shroud 15, or located radially inside the radially inner edge 15 a of the upper shroud 15, the mold is removable in the up-and-down direction.

In FIG. 7, the cover member 6 axially faces at least a portion of the lower end surface 13 a of the blade portion 13 and at least a portion of the upper surface 40 a of the circuit board 40. In at least one embodiment, the upper surface 6 c of the cover member 6 axially faces a portion, located radially outside the lower shroud 17, of the lower end surface 13 a of the blade portion 13. The lower surface 6 d of the cover member 6 axially faces a portion of the radially outer side of the upper surface 40 a of the circuit board 40. Thus, the air flowing in the axial direction is guided from the air intake port 2 a and the gap between the outer peripheral surface of the boss portion 11 and the inner peripheral surface of the upper shroud 15, in the centrifugal direction along the upper surface 6 c of the cover member 6. In at least one embodiment, regardless of the fact that the radially outer edge 17 a of the lower shroud 17 and the inner circumferential surface 4 a of the board accommodating portion 4 are separated radially due to enlargement of the circuit board 40, the air can be efficiently guided centrifugally by the cover member 6. In other words, according to at least one embodiment, by providing a wind guiding function to the cover member 6, the radial length of the lower shroud 17 is shortened, and the mold structure or the manufacturing process of the impeller 10 is simplified.

In FIGS. 7 and 8, the radially inner edge 6 e of the cover member 6 is positioned radially inside the radially outer edge 17 a of the lower shroud 17. The upper surface 6 c of the cover member 6 faces the lower surface 17 b of the lower shroud 17 in the axial direction and defines a gap therebetween. That is, the cover member 6 and the lower shroud 17 overlap each other in the axial direction. This arrangements helps to inhibit the air flowing from the air intake port 2 a to the cover member 6 along the lower shroud 17 to pass through the radially inner edge 6 e of the cover member 6. That is, according to this configuration, the air flowing from the lower shroud 17 to the cover member 6 can be inhibited from flowing into the motor side (radially inward).

In at least one embodiment, the upper surface 6 c of the cover member 6 face the lower surface 17 b of the lower shroud 17 via a gap. Thereby, the impeller 10 can be rotated smoothly.

In at least one embodiment, as in FIG. 8, the inner circumferential surface 4 a of the board accommodating portion 4 and the radially outer edge 40 b of the circuit board 40 face each other with a first gap 50 therebetween. The lower surface 6 d of the cover member 6 and the upper surface 40 a of the circuit board 40 face each other with a second gap 51 therebetween. The space above the cover member 6 in the axial direction communicates with the space in the board accommodating portion 4 located below the circuit board 40 in the axial direction, via the first gap 50 and the second gap 51. This arrangement helps to prevent heat from remaining in the space located below the circuit board 40 in the axial direction. That is, arranging electronic components on the lower surface of the circuit board 40 is easier. In at least one embodiment, the inner circumferential surface 4 a of the board accommodating portion 4 and the radially outer edge 40 b of the circuit board 40 face each other in the radial direction. The lower surface 6 d of the cover member 6 faces the upper surface 40 a of the circuit board 40 in the axial direction. Further, the lower surface 6 d of the cover member 6 may be in contact with the upper surface 40 a of the circuit board 40. That is, the second gap 51 may be omitted.

In FIG. 8, the lower shroud 17 has an inclined portion 171. The inclined portion 171 is inclined axially downward from the inside to the outside in the radial direction. In detail, the radially inner edge of the inclined portion 171 is connected to the outer peripheral surface of the boss portion 11. From the connecting portion, the inclined portion 171 extends obliquely downward. While the inclined portion 171 may have a straight shape in a cross-sectional view, the inclined portion 171 is curved in at least one embodiment. Specifically, the inclined portion 171 is concavely curved. By providing the inclined portion 171, the flowing direction of the air flowing axially from the air intake port 2 a is efficiently changed to the direction along the upper surface 6 c of the cover member 6.

In FIG. 8, the lower shroud 17 has a flat portion 172. The flat portion 172 continues to the radially outer side of the inclined portion 171. The flat portion 172 extends along a plane perpendicular to the axial direction. Specifically, the flat portion 172 is parallel to the upper surface 6 c of the cover member 6. By providing the flat portion 172 continuing to the inclined portion 171, the flowing direction of the air is smoothly changed to a direction along the upper surface 6 c of the cover member 6. In at least one embodiment, that the flat portion 172 may be omitted. In at least one embodiment, the cover member 6 includes a first portion contacting the flange portion 5 having a first thickness; and a second portion extending radially inward from the first portion having a second thickness less than the first thickness. In at least one embodiment, a top surface of the first portion is coplanar with a top surface of the second portion.

FIG. 9 is a view for explaining a centrifugal fan 100A according to at least one embodiment of the present disclosure. FIG. 9 is an enlarged sectional view similar to FIG. 8. The centrifugal fan 100A according to FIG. 9 has a configuration different from that of FIG. 8 in the relationship between a cover member 6A and an impeller 10A. The cover member 6A is fixed to a flange portion 5A of a lower casing 3A. A circuit board 40A is accommodated in a board accommodating portion 4A.

As in FIG. 9, the cover member 6A faces, in the axial direction, a portion of an lower end surface 13 aA of the blade portion 13A and a portion of an upper surface 40 aA of the circuit board 40A. However, in at least one embodiment, the cover member 6A does not face a lower surface 17 bA of a lower shroud 17A in the axial direction. In the present modification, a radially inner end surface 6 eA of the cover member 6A faces a radially outer end surface 17 aA of the lower shroud 17A in the radial direction. In at least one embodiment, the lower shroud 17A has an inclined portion 171A and a flat portion 172A. The radially outer end surface 17 aA of the lower shroud 17A is a radially outer end surface of the flat portion 172A.

In at least one embodiment, the radially inner end surface 6 eA of the cover member 6A and the radial outer end surface 17 aA of the lower shroud 17A do not come into contact with each other, i.e., there is a gap between radially inner end surface 6 eA and radial outer end surface 17 aA. In at least one embodiment, the gap in the radial direction between radially inner end surface 6 eA and radial outer end surface 17 aA is small. By avoiding contact between the radially inner end surface 6 eA and radial outer end surface 17 aA, the impeller 10A can be smoothly rotated. By narrowing the interval, the amount of air flowing in a direction different from the centrifugal direction can be reduced. The radially inner end surface 6 eA of the cover member 6A and the radially outer end surface 17 aA of the lower shroud 17A are flat. In at least one embodiment, at least one of the radially inner end surface 6 eA of the cover member 6A or the radially outer end surface 17 aA of the lower shroud 17A may be curved. In the case where the end faces 6 eA and 17 aA are planar, the planes may be parallel to the axial direction or inclined with respect to one another.

In at least one embodiment, the air flowing axially from the gap between the outer peripheral surface of the boss portion 11A and the inner peripheral surface of the upper shroud 15A can be guided in the centrifugal direction along the upper surface 6 cA of the cover member 6A. In at least one embodiment, regardless of the fact that the radially outer edge 17 aA of the lower shroud 17A and the inner circumferential surface 4 aA of the board accommodating portion 4A are separated radially due to enlargement of the circuit board 40A, the air can be guided efficiently in the centrifugal direction by the cover member 6A. In other words, according to at least one embodiment, by providing a wind guiding function to the cover member 6A, the radial length of the lower shroud 17A is shortened, and the mold structure or the manufacturing process of the impeller 10A is simplified. Furthermore, in at least one embodiment, as the cover member 6A and the lower shroud 17A do not overlap axially, the thickness of the centrifugal fan 100A can be reduced.

Various modifications can be made to the various technical features disclosed in this specification within the scope not deviating from the gist of the technical creation. Also, embodiments and modifications described herein may be implemented in combination as far as possible.

The present disclosure can be used for a centrifugal fan used for a range hood fan, a ventilating fan for a duct, a heat exchanging unit, paper adsorption for a printing apparatus, or the like. 

What is claimed is:
 1. A centrifugal fan comprising: a motor having a rotor configured to rotate about a center axis; an impeller fixed to the rotor, wherein the impeller is configured to rotate together with the rotor, and the impeller comprises: a boss portion fixed to the rotor; a plurality of blade portions, wherein adjacent blades of the plurality of blades are arranged at intervals in a circumferential direction on a radially outer side of the boss portion, and each of the plurality of the blade portions extends radially outward; an upper shroud connected to a first portion of each of the plurality of blade portions; and a lower shroud connected to a second portion of each of the plurality of blade portions, wherein the first portion is opposite the second portion; a circuit board electrically connected to the motor; and a casing accommodates the motor, the impeller, and the circuit board, wherein the casing includes a lower casing, and the lower casing comprises: a board accommodating portion recessed away from the impeller in a direction parallel to the center axis, the board accommodating portion accommodating the circuit board; and a cover member, wherein a first surface of the cover member and a surface of each of the plurality of blade portions defines a first gap, a second surface of the cover member and a surface of the circuit board defines a second gap, and the first surface is opposite the second surface.
 2. The centrifugal fan according to claim 1, wherein the lower casing has a flange portion extending radially outward from an outer peripheral edge of the board accommodating portion.
 3. The centrifugal fan according to claim 2, wherein the cover member is fixed to the flange portion.
 4. The centrifugal fan according to claim 1, wherein a radially inner edge of the cover member is positioned radially inside a radially outer edge of the lower shroud.
 5. The centrifugal fan according to claim 4, wherein an first surface of the cover member is between a portion of the lower shroud and the circuit board.
 6. The centrifugal fan according to claim 1, wherein a radially inner end surface of the cover member overlaps with a radially outer end surface of the lower shroud in a radial direction.
 7. The centrifugal fan according to claim 1, wherein the lower shroud has an inclined portion inclined toward the circuit board from an inside toward an outside in a radial direction.
 8. The centrifugal fan according to claim 7, wherein the lower shroud has a flat portion extending radially outside of the inclined portion, and the flat portion extends perpendicular to the center axis.
 9. The centrifugal fan according to claim 1, wherein a radially outer edge of the lower shroud is coplanar with a position of a radially inner edge of the upper shroud in a radial direction.
 10. The centrifugal fan according to claim 1, wherein a radially outer edge of the lower shroud is radially inward from the radially inner edge of the upper shroud.
 11. The centrifugal fan according to claim 1, wherein an inner peripheral surface of the board accommodating portion and a radially outer edge of the circuit board define a third gap.
 12. The centrifugal fan according to claim 11, wherein the first gap is in communication with the third gap.
 13. The centrifugal fan according to claim 1, wherein the casing has an upper casing, and the impeller is between the circuit board and the upper casing.
 14. The centrifugal fan according to claim 13, wherein the upper casing has an air intake port facing a radially center portion of the impeller.
 15. The centrifugal fan according to claim 1, wherein the boss portion has a cylindrical shape.
 16. The centrifugal fan according to claim 1, wherein the upper shroud has an annular shape.
 17. The centrifugal fan according to claim 1, wherein the lower shroud has an annular shape.
 18. A centrifugal fan comprising: a motor having a rotor configured to rotate about a center axis; an impeller fixed to the rotor, wherein the impeller is configured to rotate together with the rotor, and the impeller comprises: a boss portion fixed to the rotor; a plurality of blade portions, wherein adjacent blades of the plurality of blades are arranged at intervals in a circumferential direction on a radially outer side of the boss portion, and each of the plurality of the blade portions extends radially outward; an upper shroud connected to a first portion of each of the plurality of blade portions; and a lower shroud connected to a second portion of each of the plurality of blade portions, wherein the first portion is opposite the second portion; a circuit board electrically connected to the motor; and a casing accommodates the motor, the impeller, and the circuit board, wherein the casing includes a lower casing, and the lower casing comprises: a board accommodating portion recessed axially away from the impeller, the board accommodating portion accommodating the circuit board; and a cover member extending radially inward from an outer surface of the board accommodating portion, wherein the cover member is between at least a portion of the circuit board and the impeller.
 19. The centrifugal fan according to claim 18, wherein a radially outer edge of the lower shroud is coplanar with a position of a radially inner edge of the upper shroud in a radial direction.
 20. The centrifugal fan according to claim 18, wherein a radially outer edge of the lower shroud is radially outward from the radially inner edge of the cover member. 